We examined PMN and erythrocytes of patients recovering from infection for the presence of the C3b receptor, CR1, using three techniques that together measure both antigenic and functional activity. We found that CR expression was markedly reduced on PMN of patients with chronic granulomatous disease and on patients with hyper-IgE or Jobs syndrome. In contrast, other infected patients studied as controls expressed significantly more PMN CR1 than did normal healthy controls. Interestingly erythrocytes of CGD and infected control patients expressed normal levels of CR1 whereas patients with Job's syndrome were abnormally low. The C3b receptor is thought to be very important in allowing for binding and phagocytosis of opsonized bacteria. These experiments raise the possibility that these patients, with frequent infections, have an important defect in phagocytosis. The significance of these differences in CR1 expression with regard to receptor-mediated cell function is now under study.